Many disorders of the eye, a common one of which is glaucoma, cause changes in the color and/or shape of regions of the surface lining the inside of the eyeball, which is the ocular fundus. Other disorders of the entire body, such as diabetes and atherosclerosis, also produce such changes in the ocular fundus. These changes usually occur at the early stages of the disorders, and often the disorders are first detected during an examination of the ocular fundus known as a fundoscopic examination.
The detection of disorders by the fundoscopic examination may be divided into three aspects. These are (1) detection of abnormalities or changes in the color of regions of the ocular fundus, (2) detection of abnormalities or changes in the two dimensional shape of regions of the ocular fundus, e.g., changes in the tortuosity of fundus blood vessels that occur in atherosclerosis or changes in the size of a discolored region, and (3) detection of abnormalities or changes in the third dimension of regions of the ocular fundus, i.e., along the optic axis or visual axis of the eye.
Abnormal color of a region of the ocular fundus is a consequence of physical abnormalities in the fundus that cause the region to reflect light abnormally. For example, the leakage of certain chemicals from the blood into the tissue of the fundus can change the chemical composition of the region. Therefore, the way in which the region interacts with light may be changed so that, for example, a greater fraction of long wavelength light and a smaller fraction of short wavelength light are reflected. If so, the region will appear abnormally more red.
As another example, there are regions of the ocular fundus that should normally appear reddish. These regions constitute a white surface, i.e., a surface that reflects equally most wavelengths of the visible spectrum, overlaid by a fine mesh of tiny blood vessels which circulate blood, i.e., a substance that reflects long wavelength light much more strongly than short wavelength light. Abnormal color of these regions may be a consequence of impaired blood circulation. If so, if illuminated with white light, these regions will appear abnormally more white and exhibit pallor.
Various ophthalmological instruments have been developed to examine the ocular fundus to detect abnormal color, as well as changes in the two-dimensional and three-dimensional shape of the fundus. One instrument, called an ophthalmoscope, illuminates the inside of a patient's eye and provides an optical path for the physician to see the fundus. The physician relies on his own or subjective color vision to detect subtle abnormalities in the color of the fundus. This is neither highly accurate nor objective since the physician cannot visually determine the amount or intensity of light being reflected at each wavelength, which is data that provide more subtle color information. Furthermore, because many disorders are detected or their treatments monitored by evaluating changes in the fundus over time, the physician using the ophthalmoscope may have to depend either on her memory or her sketches of the appearance of the fundus. The same problems occur when evaluating changes in the two-dimensional and three-dimensional shapes of the fundus.
Another instrument is a special camera called a fundus camera that is used to photograph the ocular fundus. A series of black and white photographs can be taken with the fundus camera using light at various wavelengths. In essence, the series of photographs taken at different wavelengths represent a set of "reflectance spectra", that is, they represent the amount of light reflected from each point in the picture at each wavelength. Then, the photographs are compared visually or with a densitometer. While the reflectance spectrum is more objective data than can be obtained using the ophthalmoscope, this photographic procedure is awkward and suffers seriously from the fact that slight variations in the conditions under which the black and white film is developed can cause significant changes in the resulting photographic densities.
Also, the fundus camera can be used to take photographs over time. While these photographs provide better and more objective data than the above-mentioned sketches for detecting changes in color, as well as shape, the variability of the conditions of illumination, and especially of the chemistry of the photographic film processing, makes accurate measures of change difficult.
Furthermore, and with respect to abnormalities in the three-dimensional shape of the ocular fundus, during the course of, for example, undetected or uncontrolled glaucoma, a region of the ocular fundus called the optic disk develops cupping or excavation. That is, the surface of the optic disk recedes from the front of the eye. On the other hand, brain tumors, for example, can cause the optic disk to bulge toward the front of the eye, as can fundus tumors.
The three-dimensional shape and changes in this shape of the fundus are very difficult to evaluate with the two-dimensional photographs taken with the standard fundus camera. A modified fundus camera has been used that produces stereo pairs of fundus photographs. When viewed with a stereo viewer, these stereo pictures provide better data on which to evaluate the three-dimensional shape of the optic disk. However, the evaluation is essentially visual and subjective and, therefore, not highly accurate.